A single soil infestation with 2 and 20 macroconidia/cm 2 of soil of the mycoparasite Sporidesmium sclerotivorum was conducted in the fall of 1995 or the spring of 1996 at three Iowa locations. Soil infestation with the mycoparasite resulted in 50 to 60% reductions (P < 0.05) of Sclerotinia stem rot (SSR) of soybean in one location in 1996, and two locations in 1998. SSR was completely suppressed at a third location in 1998. In some instances the 2 spores/cm2 infestation rate provided disease control equal to the higher rate. S. sclerotivorum was retrieved from all infested plots using sclerotia of S. sclerotiorum as bait, two years after infestation, and from all control plots in the location where SSR was suppressed. Infestation of S. sclerotivorum into 100 m2 macroplots in commercial fields provided from 50 to 100% control of SSR two years later. Optimum conditions for germination of S. sclerotivorum macroconidia in the presence of sclerotia of S. sclerotiorum are pH 5.0 to 6.5, temperatures from 20 to 30°C, CO2 concentrations of 1% (v/v) or lower and O2 concentrations 1% and above. Almost no germination occurred when spores were incubated at 10 or 35°C. CO2 concentrations of 3 to 6% (v/v) reduced germination by half (P < 0.05). Baiting of 810 soil samples from 55 Iowa counties with sclerotia of Sclerotinia sclerotiorum resulted in Sporidesmium sclerotivorum isolates from soil samples from ten counties. This is the first report on the presence of this mycoparasite in Iowa. Trichoderma hamatum, T. harzianum, T. koningii, T. longibrachiatum, T. polysporum, T. pseudokoningii, and T. viride were also isolated from the sclerotial baits. Pathogenicity tests confirmed the parasitic activity of T. polysporum on the sclerotia of S. sclerotiorum. This is the first report on this association. Gliocladium roseum and G. virens were also retrieved from sclerotial baits. Coniothyrium minitans was isolated from soil samples from 12 counties, most of them located in northern Iowa.